The thermal working of workpieces includes, particularly, the welding, cutting and surface treatment for producing marks. Depending on the task to be performed, different working machines, different types of working tools, such as autogenous torches, plasma torches, lasers, or markers, are used.
Online process monitoring gains more and more importance in the thermal treatment of materials for performing high-quality welding, cutting and marking work on workpieces. For such automating operations the working processes must be monitored to avoid process errors that would lead to failure or to an interruption in the process sequence. Moreover, attention must be paid to a high and, if possible, constant quality of a cut, marking or weld seam. In this connection a uniform working distance between the working tool and the workpiece plays an important role. Known are various distance measuring and controlling devices that work in an inductive, capacitive or manual way.
A method for the capacitive distance measurement is e.g. known from DE 41 32 651 C. The capacitive sensors are arranged in concentric or offset fashion relative to the working tool, for instance, in the form of measuring elements which are mounted on the tool and are arranged opposite the workpiece to be treated. The distance between the measuring element and the workpiece is equal to a specific capacitance which is measured electronically and used for distance control.
The capacitive measurement of the distance between workpiece and working tool has, however, the drawback that the capacitive measurement value is affected by parasitic capacitances, such as rust adhering to the workpiece, surface coatings, water, water vapor, or changing ambient conditions. The measurement value is also affected by the interfering contour of the sensor element mounted on the working tool, or its holder. These interfering effects are even intensified when the working tool and the components around the tool are soiled by material rebound during the treatment of the material.
Use is also made of working machines in which the working area is surrounded by a water jacket to prevent environmentally harmful substances from exiting. The resulting pool of conductive water also leads to a wrong measurement in the case of a capacitive distance measurement. An underwater detection of data with capacitive sensors is thus not possible.
It is further known that inductive sensors are used for distance measurement, with one or several induction coils being arranged around the torch, of which each as a frequency-determining element is connected within a resonant circuit, the latter effecting a change in frequency due to a change in inductivity of the coil as a consequence of a change in distance relative to the tool. The evaluation of such frequency changes is carried out by means of known circuits or band filters.
In inductive sensors, the same problems as the ones described above for capacitive sensors arise with respect to disturbances. Moreover, inductive sensors have the drawback that they are only suited for measuring short working distances because they cannot withstand high thermal loads and can thus not be arranged in the direct neighborhood of the working process.
A method and a thermal working machine as well as a working tool according to the above-mentioned type are known from DE 37 23 844 A1. The working distance between the cutting nozzle of a welding torch and the workpiece to be treated is controlled by means of a sensor in the form of a magnet system. The sensor comprises an annular body consisting of an iron material, which surrounds the torch head and comprises four downwardly extending magnetic yokes that are distributed in concentric and uniform fashion around the circumference of the cutting nozzle. Each of the four yokes carries a coil having an axis extending in parallel with the longitudinal axis of the torch. Two opposite coils are connected in series in the same sense and are used as exciting coils in that an alternating current of high frequency is flowing through the coils, whereby an alternating magnetic field is produced in the sensor body. The second pair of opposite coils is a pair of measuring coils in which an alternating current is induced by virtue of the alternating magnetic field.
On the precondition that the magnetic alternating field which is composed of the magnetic yokes is attenuated by the workpiece, the currents induced in the measuring coils are influenced by the topography and distance of the workpiece positioned underneath the sensor. Apart from the distance between the cutting nozzle and the workpiece, this attenuation depends on the material characteristics of the workpiece, such as magnetic permeability and the electrical conductivity of the workpiece.
The distance of the cutting nozzle of the welding torch from the workpiece surface is thus a function of the voltage induced in the measuring coils, so that it can be converted in a downstream electronic system with the help of a characteristic line, which has been drawn up before for the respective process, into a voltage which is proportional to the distance and which is used as a control variable for distance control. In the control device, the respective characteristic lines and the typical set values for the working distances can be stored for different topographies and materials.
In the known device, the magnetic yoke and thus the exciting and measuring coils must be positioned near the workpiece to be treated. At said place they are exposed to a high thermal load. The intended function of the apparatus may thus be affected by the high temperature or also by material rebound from the workpiece treated.
The separate magnetic yoke which surrounds the cutting torch proper produces a large measuring patch, which is tantamount to a low measuring accuracy. It has therefore been found that the evaluation of the distance measurement in the known method is prone to a great measuring error.
Upon changes in the process parameters, e.g. the material to be treated, its topography, or upon change of the treatment tool, a new characteristic line which must be prepared under great efforts is needed for controlling the working distance.